Already known in the prior art is a three-way catalytic converter which can clean three major toxic components (HC, CO and NO.sub.x) in the exhaust gas. The operation of the three-way catalytic converter is effectively attained when the exhaust gas is in a state wherein excess air as well as excess fuel is not left in the exhaust gas, i.e., the air-fuel ratio of the exhaust gas (the ratio of air remaining in the exhaust gas to fuel remaining in the exhaust gas) is maintained near the stoichiometric value.
For maintaining the air-fuel ratio near to the stoichiometric value, an air injection system having a vacuum-operated flow control valve has been provided in the prior art for increasing or decreasing the amount of secondary air introduced into the engine exhaust manifold by selectively introducing an engine vacuum signal into a vacuum chamber of the flow control valve in accordance with electrical signals transmitted from an O.sub.2 sensor disposed in the exhaust pipe.
In order to prevent the occurrence of the so-called "overshoot" due to the ON-OFF operation of the flow control valve, it is necessary to control the rate of increase and the rate of decrease in the amount of secondary air. Therefore, the system has an orifice means for maintaining a slow rate of increase or decrease of the vacuum level in the vacuum chamber of the flow control valve.
However, in such a system, it is advantageous to control the amount of secondary air so that a rapid increase or decrease of a small amount of secondary air occurs when the flow control valve operates to begin to increase or decrease the amount of the secondary air, because such a rapid increase or decrease of the small amount of air causes the air-fuel ratio to be quickly controlled to the stoichiometric value, thus making it possible for an idealized operation of the three-way catalytic converter to occur. This rapid control of the limited amount of secondary air is required when the engine is operating under an acceleration condition because the secondary air directed into the flow control valve from an air pump driven by a crankshaft of the engine is relatively small due to the low rotational speed of the crankshaft during such an accelerating condition. The low rotational speed of the air pump causes the amount of secondary air to be increased slowly, therefore, a quick adjustment of the air-fuel ratio to the stoichiometric value cannot be carried out. As a result, the three-way catalytic converter cannot operate to clean the three major toxic components in the exhaust gas.